What are the implications for antiviral resistance among common influenza strains?

April 2006

Recently, the CDC issued a recommendation that the adamantane class of antivirals should not be used for prevention or treatment of influenza in the 2005-2006 flu season (www.cdc.gov/flu/han011406.htm). This is based on an amantadine resistance rate of 91% in the first 109 isolates this season. Testing of subsequent clinical samples has further confirmed this. Now, with more than 200 specimens tested, resistance rates remain at 92%. No isolate this season was tested resistant to the neuraminidase inhibitors, and this newer class of agents was recommended for influenza treatment and prophylaxis for the remainder of the season.

What remains to be determined is whether this resistance will continue in future influenza seasons and whether neuraminidase resistance will emerge with more widespread use of these agents. This article will briefly review the mechanisms of antiviral resistance in influenza and its implication on clinical management.

Adamantine resistance

In the adamantane class there are two agents: amantadine and rimantadine. They exert antiviral activity by blocking the influenza M2 channel protein, thereby inhibiting viral replication. The adamantanes are only active against three strains of influenza A: H1N1, H2N2 and H3N2. They are not effective for influenza B. Both agents are approved for prophylaxis and treatment of influenza A in children aged 1 year and older and adults, and are available in oral formulations. When contrasting these agents, rimantadine has two to eight times greater antiviral activity than amantadine. Therefore, amantadine is now more commonly used for treatment of Parkinson’s disease and treatment of extrapyramidal adverse events associated with other drugs.

When reviewing resistance from a historical perspective, global resistance to the adamantanes has increased from 0.4% to 12.3% in the past 11 years. Resistance increased from 1.9% to 14.5% in the past year in the United States. When examining patterns of viral respiratory illness in the United States, it is always prudent to include Asian data. Historically, Asian influenza strains have been predictive of future experiences in the United States. Adamantane-resistant influenza in China began to increase in 1997-1998 and was reported at 57.5% in 2002-2003 and 73.8% in 2003-2004. The adamantanes are available in over-the-counter products in China, and misuse of these agents may be a contributing factor to this rise in resistance. There is also documentation of infection with adamantane-resistant influenza virus in patients with no prior exposure to the adamantanes, indicating that resistant strains can be spread. This will limit the use of the adamantanes in an influenza pandemic.

Neuraminidase resistance

There are two neuraminidase inhibitors: oseltamivir (Tamiflu, Roche) and zanamivir (Relenza, GlaxoSmithKline). As their class name suggests, they inhibit influenza neuraminidase, which alters viral particle aggregation and release. Oseltamivir is approved for prevention and treatment of influenza A or B in children aged 1 year and older and adults. It is available in capsules and oral suspension, and the most commonly reported adverse events are gastrointestinal.

Zanamivir is approved for treatment of influenza A or B in children aged 7 years and older and adults. It is being studied in clinical trials for prophylaxis. This agent is unique in that it is administered by inhalation. However, this may trigger respiratory problems in patients with chronic lung disease or asthma. For treatment of influenza, these drugs must be started within two days of onset of symptoms, and the total duration of therapy is five days.

Resistance to the neuraminidase inhibitors, although not as prevalent as in the adamantane class, has been described, and is summarized in the table. However, there are differences in oseltamivir and zanamivir regarding resistance mechanism. Both agents bind to neuraminidase and inhibit its function. However, oseltamivir has a bulky side chain, which requires molecular rearrangement of the pocket surrounding the binding site on neuraminidase. Three of the neuraminidase inhibitor resistance mutations (R292K, N294S and H274Y) inhibit molecular rearrangement, therefore, preventing binding of oseltamivir. Zanamivir does not have this side chain and does not require molecular rearrangement for binding; thus, it is not affected by such mutations. A fourth mutation (E119V) has also been described in oseltamivir. This allows a water molecule to block binding of oseltamivir to the binding site. This mutation also does not affect zanamivir. Of note, in vitro experiments have generated strains of influenza resistant to both oseltamivir and zanamivir.

To better characterize the potential effects in human disease, neuraminidase resistance has been studied in animal models of infectivity. In ferrets infected with H3N2, the R292K mutation resulted in decreased growth, infectivity and transmissibility as compared with wild-type virus, indicating that this mutation may affect viral fitness. The H274Y strain was also affected, showing transmissibility only at a viral dose of 120 times that of wild-type. However, mutation with E119V did not appear to alter viral transmission among ferrets, indicating that this strain might maintain viral fitness. An interesting comparison of amantadine and zanamivir in H3N2-infected ferrets showed that amantadine resistance developed within six days, while no zanamivir resistance was detected, further supporting the durability of the neuraminidase inhibitors as compared with adamantanes.

Development of oseltamivir resistance has also been described in humans in non-H5N1 influenza strains. In one Japanese study, 10 of 182 (5.5%) children treated with oseltamivir developed resistance, eight with the R292K mutation. Another Japanese study found resistant strains in nine of 50 (18%) children after oseltamivir therapy. Six of the nine mutations were R292K. It should be noted that resistance in these studies might be due to inadequate dosing. No resistance developed in a trial of oseltamivir in U.S. children. This trial used a higher dose than the Japanese trials.

Avian influenza

The emergence of the H5N1 avian influenza strain has exceptional implications for antiviral therapy. It is a particularly virulent strain of influenza in humans, as it is theorized that humans have not had exposure to this strain and thus have no natural immunity against it. The role of antivirals is elevated to utmost importance against this strain. As noted above, the adamantane class of antivirals is subject to rapidly developing resistance and likely will not be useful for prophylaxis or treatment of H5N1. The neuraminidase inhibitors have been used for prophylaxis and treatment of H5N1 in humans with varying success. However, resistance to oseltamivir has been described previously and is of concern in this situation.

Case reports from Vietnam have described oseltamivir resistance in H5N1 strains of influenza. A 13-year-old girl received a total of 225 mg of oseltamivir on day 1 of treatment, followed by 75 mg twice daily. Her viral polymerase chain reaction (PCR) from day 4 of treatment showed H5N1 influenza with a H274Y mutation, which confers high-level resistance to oseltamivir. This patient died on day 8 of her illness. The same institution also describes a second case of oseltamivir-resistant H5N1 isolated from a patient who did not survive the illness. These results documented two cases of oseltamivir resistance out of eight patients with viral PCRs during their illness.

Another case report from Vietnam describes H5N1 infection in a 14-year-old Vietnamese girl. She had cared for her brother during his H5N1 illness and received oseltamivir 75 mg daily for 3 days of prophylaxis. She then developed symptoms, and her oseltamivir dose was increased to 75 mg twice daily for 7 days. Her influenza virus exhibited a H274Y mutation, and was an identical clone to her brother’s virus. This resulted in two unresolved issues: Did she contract a resistant virus from her brother, or did resistance develop while she was receiving oseltamivir prophylaxis? Each has its own implication. If she contracted the resistant virus from her brother, this would be evidence that H5N1 may be transmissible from human-to-human, and that resistant virus remains pathogenic and transmissible. If she developed resistance while on oseltamivir prophylaxis, this may have ramifications for the future use of oseltamivir for prophylaxis in the instance of an influenza pandemic.

It is theorized that treatment of H5N1 influenza will require higher doses of oseltamivir for longer periods. Use of current treatment doses (75 mg q12h) may result in resistance. However, the optimal dose and length of treatment are unknown at this point.

As noted above, zanamivir is not affected by mutations that confer resistance to oseltamivir. There are no known cases of zanamivir resistance in immunocompetent patients. In an animal study, activity of oseltamivir and zanamivir was compared in oseltamivir-sensitive and -resistant strains of influenza. Treatment with oseltamivir decreased viral titers in animals infected with the oseltamivir-sensitive strain, but not in the animals infected with oseltamivir-resistant strains. However, zanamivir showed activity against both strains of influenza and reduced viral titers in the animals. There are several considerations for use of zanamivir, especially for the treatment of H5N1 influenza. First, zanamivir is only available in an inhaled form, which may be difficult or impossible for patients to use if severely ill. Second, achieving therapeutic drug levels will be critical for treatment of H5N1 influenza. Medications administered by inhalation have erratic absorption, especially in the presence of structural lung disease. These concerns may be barriers to use of zanamivir in resistant influenza.

In summary, influenza resistance is an ever-increasing issue. The resistance pattern of the current influenza strain prohibits use of the adamantanes for prophylaxis or treatment, leaving the neuraminidase inhibitors as the only option for this flu season. Case reports of oseltamivir-resistant influenza have been reported, and it is anticipated that this problem may increase. With the threat of H5N1 influenza, describing resistance to our only line of defense is imperative.

For more information:

• Bright RA, Shay DK, Shu B, et al. Adamantane resistance among influenza A viruses isolated early during the 2005-2006 influenza season in the United States. JAMA. 2006;295:891-894.
• Moscona A. Neuraminidase inhibitors for influenza. N Engl J Med. 2005;353:1363-1373.

• Michelle Turner is a specialty resident in the department of internal medicine/infectious diseases at Campbell University School of Pharmacy, Duke University Medical Center.